I for one am planning to take advantage of this offer and here are my calculations hoping they are right.
I'm planning a trip to Beijing, China and the cheapest Premium Economy ticket I can find is £550 one way, that's £1100 for return (September next year, though I'm sure there will be another sale before then so the price should get even lower.). CW is £1394 one way, £2788 return. It costs 40,000 miles to upgrade from WT+ to CW.
The maximum number of miles you can buy is 19,000, with this promotion (4,750 extra) in total = 23,750 for £319. I so far have 15,000 miles and a couple of flights to US planned for early next year to gain the extra miles so it looks like there's my leisure CW trip sorted to Beijing, saving a few money.
You could argue that it's always worth buying the miles if you wish to use them for upgrades WT+ to CW as in this case it would still be a win-win, saving over £1k if you were to use the standard CW fares. If I remember correctly, during the sale, the Beijing/ Shanghai CW fare was reduced to £1700? Well, if you were to buy the miles each year, this would put you around there.
Are my calculations right, making buying the BA miles worth it in my case?